Certain patients have cardiac rhythm disturbances in which contractions of either the atria or the ventricles or both occur at a rate that is so rapid that the cardiac patient finds it intolerable. Such rapid rhythm disturbances are designated as tacharrhythmias. In some of the patients who experience tacharrhythmias, the arrhythmias can degenerate into fibrillation. Fibrillation is chaotic electrical activity of either the atria or the ventricles which does not result in coordinated cardiac contractions, with disturbances of blood flow occurring. Death can be the outcome of ventricular fibrillation. The tacharrhythmias can result in a syncope.
In both tachycardia and in fibrillation, transthoracic cardioversion-defibrillation can be applied using high energy shocks, for example, 400 joules or less. In the case of tachycardia, the arrhythmia is converted to a slower, more tolerable rate. In defibrillation, the chaotic, non-coordinated electrical activity usually becomes organized, coordinated and at a tolerable rate.
Although such patients are usually on medication to control the arrhythmias, in some instances there is a medication failure. Implantable cardiac pacers have been developed specifically to bring the tachycardias under control. In addition, an implantable defibrillator has been developed to break the chaotic electrical activity found in fibrillation.
In one example of an antitachycardia implantable cardiac pacer, the automatic antitachycardia mechanism is activated only when the pacer's tachycardia threshold is exceeded. The tachycardia threshold is exceeded when each interval between a certain number of successive sensed events is shorter than the programmed value of the interval. One of five programmable antitachycardia mechanisms is activated when the tachycardia threshold is exceeded. These mechanisms include programmed burst, burst rate scanning, automatic overdrive, programmed critically timed pulses and critically timed scanning. Many others are possible. Further, the number of attempts to control the tachycardia can be programmed. This antitachycardia implantable cardiac pacer, in the absence of a tachycardia, can also function, dependent upon lead placement, as a conventional programmable atrial and/or ventricular inhibited cardiac pacer with programmable rate, output, etc.
An implantable cardiac defibrillator functions solely to break or convert fibrillation into a more normal rhythm. The defibrillator senses the electrical activity of the ventricles. If the rate of electrical activity exceeds some value, that is preset during manufacture, the defibrillator will emit a pulse of approximately 25 joules. If the pulse is ineffective, a second and a third, if needed, are delivered.
The implantable defibrillator operates in a different manner than the antitachycardia mechanism of the pacer. For example, the implantable defibrillator can classify a tachycardia whose rate is slightly higher than the manufacturer's set fibrillation detection rate, and the defibrillator then emits a high energy pulse.
Implanting both the cardiac pacer and the defibrillator circuit in the same patient is feasible only if the antitachycardia implantable cardiac pacer is protected against the high energy pulse or pulses delivered to the heart by the implantable defibrillator, and if the implantable defibrillator is not triggered when the antitachycardia pacer is operating in its antitachycardia mode. It is desirable that the implantable defibrillator be triggered only after the antitachycardia circuit has been unsuccessful in several successive attempts to bring the arrhythmia under control.
It is thus desirable to provide means for circuit protection and tissue protection during attempts to defibrillate the ventricles, and to provide means for actuating the implantable defibrillator if the tachycardia or arrhythmia is not brought under control by the antitachycardia circuit.
It is desirable that once the arrhythmia is brought under control, the cardiac pacer with its antitachycardia circuit be reactivated to resume monitoring of the heart.
It is, therefore, an object of the invention to provide a novel cardiac stimulation apparatus and method in which a cardiac pacer having an antitachycardia circuit and a defibrillation circuit are used cooperatively to control tacharrhythmias.
Another object of the present invention is to provide a cardiac stimulation apparatus that is relatively simple in construction and efficient to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.